Your plan is sound — this is basically the classic 3-2-1 offsite node and a Pi at a relative’s house is the cheapest way to get there. Some things that will save you pain:

WireGuard alone is fine, but don’t make the Pi the server. Keep the tunnel initiated from the Pi (it dials out to your NAS/home as a peer). Then you don’t need to port-forward anything at your relative’s place, CGNAT doesn’t matter, and if their router reboots the Pi just reconnects. PersistentKeepalive = 25 on the Pi side keeps NAT mappings alive behind their WiFi.

Do the first sync at home, exactly as you said. Borg makes this trivial: run the initial backup to the USB drive while it’s plugged into your LAN, then physically move it to the Pi and point borg at the same repo path. Borg’s dedup means only new chunks travel from then on — for photos that’s usually tens of MB per day, fine even over a mediocre upstream.

Practical gotchas with the Pi-zero-offsite setup:

• Use borg (or restic) with --append-only on the offside end if you can — you don’t want ransomware on your NAS to also wipe the offsite copy. Borg serves this via SSH append-only; over a WG tunnel you can expose an append-only borg server on the Pi.
• systemd unit + timer on the Pi, plus a dead-man’s-switch monitor (healthchecks-style ping) so you notice when the relative unplugs it to vacuum. Offsite backups fail silently; this is the #1 real-world failure mode.
• ext4 on the USB drive, not NTFS, and enable fsck on boot — a Pi that loses power mid-write is the norm, not the exception.
• Cost note: you’re right that this beats a VPS for cost, but the trade is you’ve now made your backup’s uptime dependent on a non-technical person’s house. The monitoring ping is what makes it actually work.

Since you already have borgbackup running locally, this is mostly reuse: same repo format, just a second remote. No new tooling needed.

Borgbackup to a Hetzner storage box is a great solution for large (> 100GB) backups.

https://www.borgbackup.org/

https://community.hetzner.com/tutorials/install-and-configure-borgbackup/

if its on zfs, you can do zfs snapshots using send and receive, which can handle encrypted incremental snapshots which cuts down on bandwidth and without having the remote drive mounted, which I found useful for an offsite system in case someone picks it up and walks away with it. There are also automated tools to handle it too if you don’t want to write your own backup scripts, see sanoid/syncoid

I’ve got your exact setup:

• running immich (and other services with valuable data) at home (on OMV!)
• backup server at family member’s house
• regular backups from here to there through Wireguard tunnel with Borgbakup

AMA!

I don’t want a compromised device on the family member’s network to compromise mine

If with “mine” you mean the pi Zero, I wouldn’t worry too much about that. Just configure the pi’s firewall to drop all traffic from the LAN (they have no business with this device anyway), except any incoming traffic from the router on the Wireguard port.

this sounds doable. i do something similar with syncthing. it depends on the internet connection speeds. see how it goes with wireguard. between two home NAT networks you might find something lile tailscale easier to set up. wouldnt recommend doing this when youre not able to drive over there and fix things occasionally. ethernet preferable for reliable connection over wifi at the remote end. if you secure the pi well and update it it should be fine. note that you probably need to trust the remote household with your data.